cific_, characters; and the three genera adduced are somewhat
isolated, implying considerable antiquity and the extinction of many
allied forms. This is important, because it affords ample time for great
changes of conditions since the structures in question originated; and
without a knowledge of these changes we can never safely assert that any
detail of structure could not have been useful. In the second place, all
three are cases of aborted or rudimentary organs; and these are admitted
to be explained by non-use, leading to diminution of size, a further
reduction being brought about by the action of the principle of economy
of growth. But, when so reduced, the rudiment might be inconvenient or
even hurtful, and then natural selection would aid in its complete
abortion; in other words, the abortion of the part would be _useful_,
and would therefore be subject to the law of survival of the fittest.
The genera Ateles and Colobus are two of the most purely arboreal types
of monkeys, and it is not difficult to conceive that the constant use of
the elongated fingers for climbing from tree to tree, and catching on to
branches while making great leaps, might require all the nervous energy
and muscular growth to be directed to the fingers, the small thumb
remaining useless. The case of the Potto is more difficult, both because
it is, presumably, a more ancient type, and its actual life-history and
habits are completely unknown. These cases are, therefore, not at all to
the point as proving that positive specific characters--not mere
rudiments characterising whole genera--are in any case useless.

Mr. Mivart further objects to the alleged rigidity of the action of
natural selection, because wounded or malformed animals have been found
which had evidently lived a considerable time in their imperfect
condition. But this simply proves that they were living under a
temporarily favourable environment, and that the real struggle for
existence, in their case, had not yet taken place. We must surely admit
that, when the pinch came, and when perfectly formed stoats were dying
for want of food, the one-footed animal, referred to by Mr. Mivart,
would be among the first to succumb; and the same remark will apply to
his abnormally toothed hares and rheumatic monkeys, which might,
nevertheless, get on very well under favourable conditions. The struggle
for existence, under which all animals and plants have been developed,
is intermittent, an